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King County removed contaminated sediment from one acre of the Elliott Bay floor offshore of Seattle's Myrtle Edwards Park and backfilled the area with clean materials to enhance the marine habitat.

This cleanup, completed in February 2008, is part of King County's continuing efforts to remove historic contamination and restore habitat in Elliott Bay.

The project site was contaminated by 40 years of combined sewer overflows (CSOs). When flows exceeded the sewer system's capacity during heavy rains, diluted sewage mixed with storm water discharged from the Denny Way Combined Sewer Overflow (CSO) outfall. The site is no longer affected by the Denny Way CSO due to a 2005 King County CSO control project which diminished the frequency of discharges and relocated the CSO outfall further offshore.

Contaminated sediment was removed by this dredge barge. Nov. 29, 2007

A mechanical dredge on a barge excavated the area and loaded a second barge with contaminated sediment. The second barge transferred the sediment to trucks and trains for transport to a landfill. Roughly 14,000 cubic yards of sediment was removed. After dredging, the excavated area was backfilled with clean sand, armor rock and habitat-enhancing gravel to match the seabed's existing grade and improve the site as shallow-water nearshore habitat.

King County is committed to protecting the environment throughout the cleanup. To reduce sediment dispersal and turbidity, King County crews used careful dredging methods that disturb the least amount of sediment possible. Transport barges carrying excavated sediment were lined with filters to limit contaminants draining into Elliott Bay. In addition, the project's timing avoided salmon migration periods. Inspectors on site monitored water quality around the work area to ensure these precautions are fully employed and sufficient.

The project site is located along the northeastern shoreline in Elliott Bay and is adjacent to Myrtle Edwards Park at the foot of Denny Way in Seattle, Washington.

The Denny Way CSO regulator station is located at 3165 Alaskan Way. The Denny Way historical CSO outfall was located along a riprap bulkhead on the shoreline. This outfall has since been removed, and a 2005 King County project now controls sewage and stormwater overflows near the shoreline.

Vicinity and location mapVicinity and location map

Denny Way mapDetail Map

Replacement materials added after dredging will improve the site as nearshore habitat. Jan. 17, 2008

Myrtle Edwards Park remains open to the public during the project. Jan. 17, 2008

King County's contractor carefully covers the dredged area with clean materials. Jan. 17, 2008

King County contractors prepare for dredging to begin. Nov. 29, 2007

Approximately 20,000 cubic yards of contaminated sediment will be removed by this dredge barge. Nov. 29, 2007

The dredging barge moves into place directly offshore of Seattle's Myrtle Edwards Park. Nov. 29, 2007

Department of Ecology Reports

Cleanup Study Reports

Closure reports

Monitoring Reports

Denny Way/Lake Union CSO Control Project Outfall Sediment Monitoring

King County is conducting a long-term sediment monitoring program for the Denny Way/Lake Union Combined Sewer Overflow (CSO) Control Project to fulfill requirements of the Biological Opinion issued by the National Marine Fisheries Service. The objectives of the Biological Opinion are to determine the nature and extent of chemicals of concern and the condition of the benthic community in the area surrounding the Denny Way CSO. The Biological Opinion required monitoring to identify any impacts from the construction and operation of two new offshore outfalls for the Denny CSO control project, and if any are found, to propose actions to address. The chemistry and benthic community data for program monitoring years 1–5 (2006–2010) and Year 10 (2015) are included in the monitoring report listed below. Although the monitoring program is not complete yet (monitoring years 15 and 20 will occur in 2020 and 2025), this report evaluates the current status of the Denny Way site.

Monitoring to date indicates the sediment contamination present in the project area is likely related to historic conditions prior to the initiation of the control project. Chemistry data do not show increasing chemical concentrations since discharges from the Elliott West Wet Weather Treatment Station began in 2005 and the benthic community data show little discernible impacts from the construction and operation of the outfalls.

Monitoring Report, April 2018